A motor vehicle typically includes a bumper bar and an energy absorber which supports the bumper bar on a body of the motor vehicle for translation though a relatively short energy-absorbing stroke in response to a low speed impact on the bumper bar. During the energy-absorbing stroke, a fraction of the kinetic energy of the impact is converted by the energy absorber into work. In a high speed impact on the bumper bar, however, its short energy-absorbing stroke is quickly traversed and most of the kinetic energy of the impact is converted into work by plastic deformation of body structure of the motor vehicle behind the bumper bar. As motor vehicles have become more compact, the energy-absorbing capability of their body structures has decreased due to the smaller span between the vehicle's passenger compartment and bumper bar. A telescoping device described in U.S. Pat. No. 5,370,429 supports a bumper bar close to a body of a motor vehicle except when sensors on the vehicle detect an impending impact. Then, the telescoping device extends the bumper bar out from the body to maximize the energy-absorbing stroke of the bumper bar. During the energy-absorbing stroke, hydraulic fluid is throttled through an orifice of the telescoping device to absorb a fraction of the kinetic energy of the impact. The telescoping device described in the aforesaid U.S. Pat. No. 5,370,429 is not "self-locking", i.e., does not become structurally rigid in compression under any circumstances, and requires a fluid reservoir and fluid seals which may leak during the service life of the device. Accordingly, manufacturers continue to seek improved telescoping devices which are self-locking and which are also suitable for use as bumper energy absorbers.